Pulping processes can be broadly classified into high yield processes using mechanical fiberizing equipment and low yield processes using chemical reactions to produce individual fibers or pulp from lignocellulosic raw materials, usually wood in chip form. Within the high yield category there are many variations which involve varying combinations of chemical, mechanical, and thermal treatments to effect fiber separation, remove some lignin and other chemical components from the original fibers, or increase the brightness and papermaking strength of the resulting fibers. This invention is directed to the art of high yield pulping in which mechanical treatment either with or without heat is the primary means of fiber separation and mild chemical treatment is used to facilitate fiber separation and to increase the papermaking strength and brightness. The application of heat may be utilized in combination with mechanical and chemical treatments to further assist in fiber separation and to accelerate chemical reactions. However, the primary goal of this invention is to economically produce pulp in the highest possible yield of the original lignocellulosic raw material by retaining and chemically modifying the lignin in the fibers to obtain the desired papermaking properties.
Mechanically refined pulp without chemical pretreatment results in extremely high yields (about 95% or higher) but results in fibers containing almost all of the original lignin in essentially a chemically unmodified form. Such unmodified lignin imparts relatively low brightness to the fibers, and, due to its hydrophobic nature, the lignin inhibits the development of paper strength through fiber to fiber bonding (hydrogen bonding) and makes the fibers much stiffer than partially or completely delignified fibers from the same lignocellulosic raw material. Although some high yield pulps containing high amounts of lignin can be bleached economically to relatively high brightnesses using oxidizing agents such as alkaline peroxide and/or reducing agents such as sodium hydrosulfite, such post-refining treatments do not increase papermaking strength to levels required for many end uses because much mechanical damage has already been done to the fibers.
The papermaking strength of high yield pulps can be increased by sulfonation of the lignin, particularly when the wood chips are treated with the sulfonation chemicals (usually sodium sulfite and sodium hydroxide) prior to mechanical defibration (refining). In some cases, the resulting fibers can also be bleached economically as with alkaline peroxide and/or sodium hydrosulfite to give both improved brightness and papermaking strength. However, the high levels of sulfonation required for high strength result in pulps which respond less to bleaching than similar non-sulfonated or low-sulfonated pulps, and therefore such highly sulfonated pulps have lower bleached brightnesses although high strength. Moreover, sulfonation processes require the removal and disposal of environmentally objectionable sulfur compounds from process waste streams. In addition, the need for separate sulfonation, refining, post-refiner bleaching, and effluent treatment equipment makes the capital equipment and operating costs for such a system very significant.
An alternative to sulfonation of lignin for increasing papermaking strength of high yield fibers is carboxylation of lignin with oxidants such as alkaline peroxide prior to and/or during defibration. As sulfonation results in lignin containing sulfonate groups, likewise, carboxylation results in lignin with carboxylate groups. Both the sulfonate and the carboxylate groups are capable of participating in hydrogen bonding which increases the strength of paper made from such high yield pulps (papermaking strength).
Similar to the alkaline sulfonation treatment of chips prior to refining, alkaline peroxide pretreatment of chips softens the lignocellulosic raw material resulting in easier fiber separation (less energy consumption) and less fines generations (fiber fragmentation) during refining. In addition, refiner bleaching with alkaline peroxide can potentially eliminate the need for separate post-refiner bleaching equipment, due to the facts that refiners are excellent mixers of pulp and bleaching agents, and the temperature within the refiner (about 100.degree. C.) causes bleaching to occur extremely fast relative to typical post-refiner alkaline peroxide bleaching steps (approximately 50.degree. to 80.degree. C.).
Offsetting the advantages is the primary drawback to alkaline peroxide refiner bleaching of peroxide decomposition. Peroxide decomposes to form oxygen (ineffective for lignin-retaining bleaching) under the highly alkaline conditions required for papermaking strength development. Peroxide decomposition is hastened by the high temperatures reached in refiners and by metal contaminants, particularly manganese, iron, and copper, which are contained in significant quantities in lignocellulosic raw materials and in lesser quantities in process water. Partial removal or inactivation of such metal contaminants in lignocellulosic raw material can be effected by introducing chelating agents into the wood chips and then removing the chelant-metal complexes. However, the physical entrapment and chemical attraction of such metals by fiber components within the chips make complete removal of the metals impossible.
The problems associated with prerefiner or in-refiner alkaline peroxide treatments are partially avoided by post refiner alkaline peroxide treatments. For example, removal of metal contaminants from individual fibers with chelating agents after refining and prior to alkaline peroxide bleaching is much more effective because the particle size of the fiber in pulp is much smaller than chips before refining. The smaller size makes the metal contaminants much more accessible to the chelant solution. Consequently, in many cases the individual fibers can be bleached to much higher brightnesses with alkaline peroxide in a post refiner bleaching treatment without significant waste of peroxide bleaching agents due to metal contaminant induced decomposition of peroxide (U.S. Pat. No. 4,160,693-Lindahl et al.).
There have been many attempts to overcome such problems associated with the pre-refiner or in-refiner use of hydrogen peroxide in the production of high yield pulps. Control of alkalinity (e.g., see U.S. Pat. Nos. 3,069,309-Fennell and 4,270,976-Sandstrom et al., and Canadian Patent Nos. 1,078,558 and 1,173,604), control of the temperature (e.g., U.S. Pat. No. 4,187,141-Ahrel), and control of time at high temperature (e.g., U.S. Pat. No. 4,270,976-Sandstrom et al.) have been tried. However, such techniques for reducing peroxide decomposition also reduce the effectiveness of alkaline peroxide in terms of the resulting pulp properties (papermaking strength) while the presence of deleterious metal contaminants still results in inefficient utilization of the peroxide bleaching agent during refiner bleaching.
Alkaline peroxide stabilizers like water soluble sodium silicate and magnesium sulfate are often utilized in the peroxide bleaching of high yield pulps to further reduce peroxide decomposition caused by metal contaminants. The silicate forms a flock in alkaline peroxide solutions and this flock attracts and adsorbs the metal ions on its surface thereby reducing their ability to decompose peroxide. Magnesium ions also reduce peroxide decomposition by electronically deactivating the metal ions, thereby reducing the potential of the metal ions to decompose peroxide. However, the present invention is based in part upon the belief that flocks or precipitates formed by silicates and/or magnesium in alkaline peroxide solutions cannot readily penetrate into the wood chip structures prior to refining due to the large size of the flock relative to the pore size of the wood chips. This belief is reinforced by the fact that such stabilizers effectively stabilize peroxide against decomposition when pulp is being bleached with peroxide but are not as effective when the wood is in chip form rather than pulp. It is believed that the difference in stabilizer effectiveness when treating pulp fibers versus wood chips is due to the alkali and peroxide entering the chip structure while the stabilizer flock is impeded from penetrating the chip with the result that the peroxide, separated from the stabilizing flock, rapidly decomposes within the chip thereby reducing the amount of peroxide available for bleaching during refining. In addition, the pressure buildup within the chip due to the evolution of oxygen gas during peroxide decomposition forces alkaline peroxide out of the chip with the result that insufficient peroxide is retained in the chip as it enters the refining zone. Furthermore, irreversible alkaline yellowing of the pulp occurs if there is insufficient residual peroxide remaining with the pulp after refining.
A common method for circumventing the problem of peroxide decomposition in alkaline peroxide bleaching of chips during refining is to add the bleaching agent directly to the refining zone to minimize the contact time between the chip and alkaline peroxide, and in some cases to allow more intimate contact between metal contaminants and silicate and/or magnesium ion stabilizer flocks (See for example, U.S. Pat. Nos. 3,023,140-Textor, 3,069,309-Fennell, 4,022,965-Goheen et al., 4,270,976-Sandstrom et al., 4,311,553-Akerlund et al.; Japanese Patent Application No. 80-72091, and Federal Republic of Germany Patent No. 2818-320). Additionally, wood chips have been pretreated by impregnation and/or refining with chelants (U.S. Pat. Nos. 3,023,140-Textor, 4,311,553-Akerlund et al., Japanese Patent Application No. 80-72091, and Federal Republic of Germany Patent No. 2818-320) or with sodium silicate (U.S. Pat. Nos. 3,069,309-Fennell, 4,311,553-Akerlund et al.), or with magnesium salts (U.S. Pat. Nos. 3,023,140-Textor, 3,069,309-Fennell, 4,311,553-Akerlund et al. and Japanese Patent Application No. 80-72091) and combinations thereof prior to alkaline peroxide addition into the refiner to reduce peroxide decomposition. U.S. Pat. No. 4,270,976-Sandstrom et al. is the only case in which brightness comparable to post refiner alkaline peroxide bleaching was obtained but it utilized lower alkalinity as the means of reducing the peroxide decomposition rate which sacrificed good papermaking strength development. The present invention is based in part upon the hypothesis that with processes employing alkaline peroxide addition directly into the refiner, the majority of defibration occurs before the alkali and peroxide contact the fibers and have an opportunity to swell and react with the wood, thereby reducing the potential for papermaking strength development, along with requiring more energy for refining and increasing the generation of fines, all of which could be avoided if alkaline peroxide could be inserted and stabilized within the chip prior to defibration.
Impregnation of the chips with alkaline peroxide prior to refining has also been practiced (U.S. Pat. Nos. 4,187,141-Ahrel, and 4,270,976-Sandstrom et al., and Canadian Patent Nos. 1,078,558, and 1,173,604). In most cases, the brightness obtained was comparable to that obtainable with post refiner alkaline peroxide bleaching. However, with such processes, metal contaminants are not removed or deactivated; rather, peroxide decomposition is reduced by lower alkalinity (U.S. Pat. Nos. 4,270,976-Sandstrom et al., Canadian Patent Nos. 1,078,558, and 1,173,604) or by minimizing refining temperature (U.S. Pat. No. 4,187,141-Ahrel). However, the lowering of the alkalinity or temperature causes less papermaking strength to be developed than sulfonation methods. At higher alkalinity, strengths comparable to those of post refiner bleached, sulfonated high yield pulps were obtained but at the expense of lower brightness due to increased peroxide decomposition (Canadian Patent No. 1,078,558).